This patch boils down to the following improvements:

* Various linkage-related shenanigans have been documented and revamped.
* All logic to build tarantool-sys is now encapsulated in tarantool-build.
* Most of the magic `println!`s are now encapsulated in build-rs-helpers.
* Unsound function `export_symbols` has been dropped in favor of
  explicit linker flags (`--require-defined` for linux & `-u` for mac).
* `TarantoolBuildRoot` is now in charge of building and linking
  tarantool libs. It defines the build tree via path helper functions.
* Picodata's `build.rs` should be a little easier to read.